This invention relates to foamed articles of ethylenic polymers. It also pertains to an improvement in a process whereby there are obtained ethylenic polymer foam planks having low density, dimensional stability, and substantially closed-cell structures.
The term "low density ethylenic polymer foam planks" as used herein means ethylenic polymer cellular structures having densities from about 0.8 to about 1.9 pounds per cubic foot (pcf), a cross-sectional area of at least 6 square inches, and a minimal cross-sectional dimension of at least 0.5 inch and such term is especially applicable herein to such cellular structures having densities from about 0.8 to about 1.6 pcf.
The term "substantially closed-cell structures" as used herein means that the foam articles thereby referred to contain less than about 20 (preferably less than about 15) percent of open cells therein.
It is well known to make closed-cell ethylenic polymer resin foams by the process of extrusion foaming wherein a normally solid thermoplastic ethylenic polymer resin such as polyethylene is heat-plastified and mixed under pressure with a volatile material such as 1,2-dichlorotetrafluorethane to form a flowable gel which is then passed through a shaping orifice or die opening into a zone of lower pressure. Upon the release of pressure, the volatile constituent of the gel vaporizes, forming a gas phase cellular structure in the gel which cools to a corresponding cellular foamed solid resin. Desirably, the resulting gas cells are substantially uniform in size, uniformly distributed through the foam body, and closed, i.e., separated from each other by membrane walls of resin. Although a number of general principles are thought to be understood, much of the extrusion foaming technology is empirical, based on experience, and directed to very specific materials and details to produce saleable products of narrowly defined specification.
One of the common requirements of acceptable foam resin products is dimensional stability, i.e., it is desired that the linear dimension and thus the volume of a piece of foam resin not change appreciably, either to shrink or to expand, under ordinary conditions, from the time its manufacture is complete until the time its ultimate useful life is ended. It is also desired that if any appreciable shrinking of a foam is to occur, which is usually the case with a freshly extruded foam, the foam be able to recover within a reasonable period of time to a substantially constant volume close to that of the foam measured shortly after its extrusion.
With respect to dimensional stability, it has been explained that the vapors of volatile material originally present in the cell gradually permeate the cell wall and escape from the foam over a period of time, thereby tending to reduce the inner cell pressure and tending to cause the foam to shrink during that time. However, when the foam is exposed to ambient atmosphere, air and its constituent gases also tend to permeate into the foam through the cell wall over a period of time thereby tending to increase the inner cell pressure. By virtue of the possibly differing relative rates of permeation of blowing agent out of and air into the cellular structure, there exists the potential for the development of a significant pressure differential (relative to ambient air pressure) within such cellular structure with attendant potential shrinking or expansion thereof during the indicated air/blowing agent interchange therein. Accordingly, the difficulties of attaining dimensional stability are particularly acute in foams of relatively low density (high expansion ratio) when the resin membrane cell walls are relatively thin.
The difficulties of attaining dimensional stability are further accentuated in relatively thick foams (i.e., foam planks) since, with such thick foams, the time required to reach substantially constant, commercially acceptable volume is relatively long, i.e., more time is required for rates of diffusion of residual blowing agent out of the foams and air into such foams to balance the pressure therein. Moreover, the problem of attaining satisfactory dimensional stability is especially severe in highly expanded (i.e., low density) foams of non-crosslinked ethylenic polymer resins since the resultant relatively thin membrane cell walls have relatively low strength with which to resist deformation (e.g., shrinkage) due to the aforementioned pressure differential occurring during the blowing agent/air interchange process.
Polyethylene foam planks having thickness greater than about 0.5 inch, dimensional stability, and substantially closed-cell structure are well known items of commerce. Well known end use applications of such planks are found in the packaging, automotive, construction, contact and water sports and appliance markets. However, the substantially closed-cell polyethylene foam planks known to date have foam density greater than 1.6 pounds per cubic foot. Moreover, in the case of substantially closed-cell polyethylene foam prepared by extrusion foaming technology employing the hereinafter described Group I C.sub.2 -C.sub.4 fluorocarbon blowing agents, foams capable of preparation by such process have heretofore been generally limited to those having densities in excess of 2 pounds per cubic foot. In spite of the economic incentive associated with more efficient use of raw materials, attempts to produce polyethylene foam planks having density lower than that indicated above using the conventional extrusion technology known in the art have met with repeated failures. As a result, foam planks of non-crosslinked polyethylene resins having thickness greater than about 0.5 inch and having foam density lower than about 1.6 pounds per cubic foot are not presently commercially available; nor is there presently commercially available an extrusion foaming process employing the indicated Group I C.sub.2 -C.sub.4 fluorocarbon blowing agents for preparing substantially closed-cell polyethylene foam planks having a density of 1.9 pcf or less. Nevertheless, there is a definite need and desire for foam planks having foam density lower than that of conventional planks for the end use applications discussed hereinabove and for a C.sub.2 -C.sub.4 fluorocarbon blowing agent-based extrusion foaming process for the preparation of such low density foams.
Accordingly, an object of this invention is to provide improved polyethylene foam planks. Another object is to provide method and means for making such foam planks. A particular object is to provide such improved method and means for making polyethylene foam planks having dimensional stability, substantially closed-cell structure, and foam density lower than that of the conventional polyethylene foam planks known to date. Other objects and advantages of the present invention are brought out in the description that follows.